Process for the production of hydrocarbons from gaseous hydrocarbonaceous feed

ABSTRACT

A process for the production of hydrocarbons from gaseous hydrocarbonaceous feed involving the steps of:
         i) partial oxidation conversion of the gaseous hydrocarbonaceous feed and oxygen containing gas at elevated temperature and pressure into synthesis gas;   ii) catalytical conversion of synthesis gas of step i) using a cobalt based Fischer-Tropsch catalyst into a hydrocarbons comprising stream;   iii) separating the hydrocarbons comprising stream of step ii) into a hydrocarbons product stream and a recycle stream; and,   iv) removing carbon dioxide from the recycle stream and recycle of carbon dioxide depleted recycle stream to step i).

PRIORITY CLAIM

The present application claims priority on European Patent Application022539126.1 filed 5 Jun. 2002.

FIELD OF THE INVENTION

The present invention relates to a process for the production ofhydrocarbons from gaseous hydrocarbonaceous feed.

BACKGROUND OF THE INVENTION

This process comprises in general the conversion of a hydrocarbonaceousfeed by partial oxidation using an oxygen containing gas into synthesisgas. Subsequently, this synthesis gas is catalytically converted intohydrocarbons using a Fischer-Tropsch catalyst.

U.S. Pat. No. 4,046,829 discloses a method for producing hydrocarbonsfrom coal using an iron based Fischer-Tropsch catalyst. Coal is gasifiedand synthesis gas formed is gas scrubbed and subsequently subjected topartial oxidation with oxygen. After the Fischer-Tropsch conversion ofsynthesis gas low hydrocarbons are separated, recycled and after carbondioxide removal mixed with synthesis gas prior to the partial oxidation.

U.S. Pat. No. 4,433,065 discloses a process for producing hydrocarbonsfrom coal using a cobalt based Fischer-Tropsch catalyst. After removalof liquid hydrocarbons the gas phase is subject to carbon dioxideremoval. After separation a hydrogen comprising stream is recycled tothe partial oxidation process, a light hydrocarbons comprising stream isrecycled to the coal gasification process, and a carbon monoxidecomprising stream is subjected to combustion for electricity generation.

U.S. Pat. No. 5,324,335 discloses a process for producing hydrocarbonsusing an iron-based Fischer-Tropsch catalyst in which hydrocarboncontaining gas is subjected to steam reforming for producing synthesisgas. After carbon dioxide removal the synthesis gas is subjected to theFischer-Tropsch conversion. Light hydrocarbons are separated, recycledand mixed with the synthesis gas.

SUMMARY OF THE INVENTION

The invention is directed to a process for the production ofhydrocarbons from gaseous hydrocarbonaceous feed comprising the stepsof:

-   i) partially oxidating the gaseous hydrocarbonaceous feed and oxygen    containing gas at elevated temperature and pressure into synthesis    gas;-   ii) catalytically converting the synthesis gas of step i) using a    cobalt based Fischer-Tropsch catalyst into a hydrocarbons comprising    stream;-   iii) separating the hydrocarbons comprising stream of step ii) into    a hydrocarbons product stream and a recycle stream; and,-   iv) removing carbon dioxide from the recycle stream and recycle of    carbon dioxide depleted recycle stream to step i).

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to a process for the production of relativelyhigh hydrocarbons using a cobalt Fischer-Tropsch catalyst. Moreparticularly, the invention concerns a cobalt catalyst, especially acobalt-zirconia catalyst, which is favorable for producing a relativelylarge amount of hydrocarbons in the C₁₀-C₁₄ range beside a lighter and aheavier fraction. This favor for C₁₀-C₁₄ hydrocarbons, especiallyunsaturated hydrocarbons, results, however, in a higher production ofoffgas when compared with a process which is optimal for the productionof the most heavy paraffinic products. In modern plant design thisoffgas may not be flared but rather is to be used or reprocessed.

According to the process of the invention, the hydrocarbons comprisingstream is separated into a hydrocarbons product stream and a recyclestream. Carbon dioxide is removed from the recycle stream and the carbondioxide depleted recycle stream is used as a feed for the partialoxidation conversion Preferably at least 70 vol. % of carbon dioxide isremoved, more preferably at least 80 vol. %, even more preferably atleast 90 vol. %. The recycle stream comprises predominantly hydrogen,carbon monoxide, C₁ to C₃ hydrocarbon, in some cases also C₄ and minoramounts of C₅₊ hydrocarbon and inerts as nitrogen noble gasses.

A reprocessing of the recycle stream without prior carbon dioxideremoval would have resulted in synthesis gas having a low H₂/CO ratiowhich is inappropriate for use in the Fischer-Tropsch conversion ofsynthesis gas into the desired hydrocarbons. Direct use of the recyclestream in the partial oxidation conversion would provide synthesis gaswith a level of inerts that is too high Removal of carbon dioxide priorto use in the partial oxidation conversion will reduce the level ofinerts in the synthesis gas produced. Use of the carbon dioxide depletedrecycle stream in turn results in the use of less oxygen in the partialoxidation conversion. The recycle stream optimizes the carbon efficiencyof the process. This in its turn increases the thermal efficiency of theprocess. Finally, removal of carbon dioxide costs less than a conversionof carbon dioxide into carbon monoxide.

According to the invention, a cobalt based Fischer-Tropsch catalyst maybe used. In particular, the catalyst may be a cobalt on zirconiacatalyst, which is favorable for the production of C₁₀-C₁₄ hydrocarbonswherein the offgas produced does not result in an extensive increase ofcosts and the amount of carbon dioxide to be removed is minimal due tothe use of gaseous hydrocarbonaceous feed which results in a lowerproduction of carbon dioxide.

The process of recycling the carbon dioxide depleted recycle stream issimplified if this carbon dioxide depleted recycle stream is firstcompressed, mixed with gaseous hydrocarbonaceous feed and subsequentlyintroduced in the partial oxidation conversion using oxygen containinggas.

In order to avoid a build-up of inerts in the process, it is preferredthat part of the recycle stream of step iii), e.g. between 5 and 50 vol.%, preferably between 10 and 40 vol %, of the total stream, is used asfuel in steam reforming of gaseous hydrocarbonaceous feed for producinghydrogen supplement for synthesis gas of step i).

Accordingly, inerts such as carbon dioxide and nitrogen are removed fromthe process after combustion as flue gas and the hydrogen or hydrogenrich synthesis gas produced in the SMR process may be used for adjustingthe H₂/CO ratio of the synthesis gas.

According to a further preferred embodiment part of the recycle streamof step iii) or step iv) is used as fuel for power generation.

Finally, it is preferred that the hydrocarbons product stream issubjected to catalytic hydrocracking. Accordingly, the molecular weightdistribution of hydrocarbons produced may be adjusted as desired.

The hydrocarbonaceous feed suitably is methane, natural gas, associatedgas or a mixture of C₁₋₄ hydrocarbons. The feed comprises mainly, i.e.more than 90 v/v %, especially more than 94%, C₁₋₄ hydrocarbons,especially comprises at least 60 v/v percent methane, preferably atleast 75 percent, more preferably 90 percent. Very suitably natural gasor associated gas is used. Suitably, any sulphur in the feedstock isremoved.

The (normally liquid or solid) hydrocarbons produced in the process andmentioned in the present description are suitably C₃₋₁₀₀ hydrocarbons,more suitably C₄₋₆₀ hydrocarbons, especially C₅₋₄₀ hydrocarbons, moreespecially C₆₋₂₀ hydrocarbons, or mixtures thereof. These hydrocarbonsor mixtures thereof are liquid or solid at temperatures between 5 and30° C. (1 bar), especially at 20° C. (1 bar), and usually are paraffinicof nature, while up to 30 wt %, preferably up to 15 wt %, of eitherolefins or oxygenated compounds may be present.

The partial oxidation of gaseous feedstocks, producing mixtures ofespecially carbon monoxide and hydrogen, can take place in the oxidationunit according to various established processes. Catalytic as well asnon-catalytic processes may be used. These processes include the ShellGasification Process. A comprehensive survey of this process can befound in the Oil and Gas Journal, Sep. 6, 1971, pp 86-90. The partialoxidation process may be carried out in combination with a reformingprocess, e.g. in the form of an autothermal reforming process.

The oxygen containing gas is air (containing about 21 percent ofoxygen), or oxygen enriched air, suitably containing up to 100 percentof oxygen, preferably containing at least 60 volume percent oxygen, morepreferably at least 80 volume percent, more preferably at least 98volume percent of oxygen. Oxygen enriched air may be produced viacryogenic techniques, but is preferably produced by a membrane basedprocess, e.g. the process as described in WO 93/06041.

To adjust the H₂/CO ratio in the syngas, carbon dioxide and/or steam maybe introduced into the partial oxidation process. Preferably up to 15%volume based on the amount of syngas, preferably up to 8% volume, morepreferably up to 4% volume, of either carbon dioxide or steam is addedto the feed. As a suitable steam source, water produced in thehydrocarbon synthesis may be used. As a suitable carbon dioxide source,carbon dioxide from the effluent gasses of the expanding/combustion stepmay be used. The H₂/CO ratio of the syngas is suitably between 1.5 and2.3, preferably between 1.8 and 2.1. If desired, (small) additionalamounts of hydrogen may be made by steam methane reforming, preferablyin combination with the water shift reaction. Any carbon monoxide andcarbon dioxide produced together with the hydrogen may be used in thehydrocarbon synthesis reaction or recycled to increase the carbonefficiency.

The percentage of hydrocarbonaceous feed which is converted in the firststep of the process of the invention is suitably 50-99% by weight andpreferably 80-98% by weight, more preferably 85-96% by weight.

The gaseous mixture, comprising predominantly hydrogen, carbon monoxideand optionally nitrogen, is contacted with a suitable catalyst in thecatalytic conversion stage, in which the normally liquid hydro-carbonsare formed. Suitably at least 70 v/v % of the syngas is contacted withthe catalyst, preferably at least 80%, more preferably at least 90,still more preferably all the syngas.

The catalysts used for the catalytic conversion of the mixturecomprising hydrogen and carbon monoxide into hydrocarbons are known inthe art and are usually referred to as Fischer-Tropsch catalysts. Thecatalysts for use in the Fischer-Tropsch hydrocarbon synthesis processcomprise, as the catalytically active component cobalt.

The catalytically active cobalt is preferably supported on a porouscarrier. The porous carrier may be selected from any of the suitablerefractory metal,oxides or silicates or combinations thereof known inthe art. Particular examples of preferred porous carriers includesilica, alumina, titania, zirconia, ceria, gallia and mixtures thereof,especially silica and titania.

The amount of catalytically active cobalt on the carrier is preferablyin the range of from 3 to 300 pbw per 100 pbw of carrier material, morepreferably from 10 to 80 pbw, especially from 20 to 60 pbw.

If desired, the cobalt based Fischer-Tropsch catalyst may also compriseone or more metals or metal oxides as promoters. Suitable metal-oxidepromoters may be selected from Groups IIA, IIIB, IVB, VB and VIB ofthe-Periodic Table of Elements, or the actinides and lanthanides. Inparticular, oxides of magnesium, calcium, strontium, barium, scandium,yttrium, lanthanum, cerium, titanium, zirconium, hafnium, thorium,uranium, vanadium, chromium and manganese are most suitable promoters.Particularly preferred metal oxide promoters for the catalyst used toprepare the waxes for use in the present invention are manganese andzirconium oxide. Suitable metal promoters may be selected from GroupsVIIB or VIII of the Periodic Table. Rhenium and Group VIII noble metalsare particularly suitable, with platinum and palladium being especiallypreferred. The amount of promoter present in the catalyst is suitably inthe range of from 0.01 to 100 pbw, preferably 0.1 to 40, more preferably1 to 20 pbw, per 100 pbw of carrier.

The catalytically active cobalt and the promoter, if present, may bedeposited on the carrier material by any suitable treatment, such asimpregnation, kneading and extrusion. After deposition of the cobaltand, if appropriate, the promoter on the carrier material, the loadedcarrier is typically subjected to calcination at a temperature ofgenerally from 350 to 750° C., preferably a temperature in the range offrom 450 to 550° C. The effect of the calcination treatment is to removecrystal water, to decompose volatile decomposition products and toconvert organic and inorganic compounds to their respective oxides.After calcination, the resulting catalyst may be activated by contactingthe catalyst with hydrogen or a hydrogen-containing gas, typically attemperatures of about 200 to 350° C.

The catalytic conversion process may be performed in the conversion unitunder conventional synthesis conditions known in the art. Typically, thecatalytic conversion may be effected at a temperature in the range of150 to 350° C., preferably from 180 to 270° C. Typical total pressuresfor the catalytic conversion process are in the range of from 1 to 200bar absolute, more preferably from 10 to 70 bar absolute. In thecatalytic conversion process preferably (at least 50 wt % of C₅+,preferably 70 wt %) C₅₋₂₀ hydrocarbons are formed.

The amount of C₁₀-C₁₄ which is directly formed in step ii) of theprocess is suitably between 12 and 27 wt % of the C₅+ product stream,preferably between 17 and 27 wt %, more preferably between 22 and 27 wt%. A high amount is preferred as the C₁₀-C₁₄ fraction is a valued LDFfeedstock.

The average ASF value for the C₅+ product stream of the step ii) of theprocess according to the present invention is suitable between 0.95 and0.80, preferably between 0.92 and 0.82, preferably between 0.90 and0.85. Higher values will result in a relative low amount of C₁₀-C₁₄fraction, lower values will result in too much C₁-C₄ products, whichproducts have a low value. The ASF value can be optimized by changingreaction conditions, especially H₂/CO ratio and temperature, but alsoGHSV and pressure, and by a suitable choice of the catalyst. Especiallya cobalt on zirconia carrier is suitable. The relative low ASF value(when compared with Fischer Tropsch processes directed to waxproduction) result in a relative large gas fraction to be recycled. CO₂removal is especially suitable under those conditions.

The process according to the present invention is especially suitablefor Fischer Tropsch plants which use a two or three stage FischerTropsch process. The relative low ASF values not only directly result ina large amount of C₁-C₄ products, but these large amounts of gas alsoresult (keeping any other variables the same) in an indirect increase ofthe C₁-C₄ fraction in the second and third stage (H₂/CO ratio and GHSV).

The cobalt based Fischer-Tropsch catalyst used, yields substantialquantities of paraffins, more preferably substantially unbranchedparaffins. A part may boil above the boiling point range of theso-called middle distillates. The term “middle distillates”, as usedherein, is a reference to hydrocarbon mixtures of which the boilingpoint range corresponds substantially to that of kerosene and gas oilfractions obtained in a conventional atmospheric distillation of crudemineral oil. The boiling point range of middle distillates generallylies within the range of about 150 to about 360° C.

The higher boiling range paraffinic hydrocarbons, if present, may beisolated and subjected in an optional hydrocracking unit to a catalytichydrocracking which is known per se in the art, to yield the desiredmiddle distillates. The catalytic hydro-cracking is carried out bycontacting the paraffinic hydrocarbons at elevated temperature andpressure and in the presence of hydrogen with a catalyst containing oneor more metals having hydrogenation activity, and sup-ported on acarrier. Suitable hydrocracking catalysts include catalysts comprisingmetals selected from Groups VIB and VIII of the Periodic Table ofElements. Preferably, the hydrocracking catalysts contain one or morenoble metals from group VIII. Preferred noble metals are platinum,palladium, rhodium, ruthenium, iridium and osmium. Most preferredcatalysts for use in the hydro-cracking stage are those comprisingplatinum.

The amount of catalytically active metal present in the hydrocrackingcatalyst may vary within wide limits and is typically in the range offrom about 0.05 to about 5 parts by weight per 100 parts by weight ofthe carrier material.

Suitable conditions for the optional catalytic hydrocracking in ahydrocracking unit are known in the art. Typically, the hydrocracking iseffected at a temperature in the range of from about 175 to 400° C.Typical hydrogen partial pressures applied in the hydrocracking processare in the range of from 10 to 250 bar.

The process may conveniently and advantageously be operated in a recyclemode or in a single pass mode (“once through”) devoid of any recyclestreams. This single pass mode allowing the process to be comparativelysimple and relatively low cost.

The recycle stream obtained after separation of the hydrocarbons maycomprise normally gaseous hydrocarbons produced in the synthesisprocess, nitrogen, unconverted methane and other feedstock hydrocarbons,unconverted carbon monoxide, carbon dioxide, hydrogen and water. Thenormally gaseous hydrocarbons are suitably C₁₋₅ hydrocarbons, preferablyC₁₋₄ hydrocarbons, more preferably C₁₋₃ hydrocarbons. Thesehydrocarbons, or mixtures thereof, are gaseous at temperatures of 5-30°C. (1 bar), especially at 20° C. (1 bar). Further, oxygenated compounds,e.g. methanol, dimethylether, may be present. For the removal of carbondioxide any suitable conventional process may be used, for instanceadsorption processes using amines, especially in combination with aphysical solvent, such as the ADIP process or the SULFINOL process asdescribed in inter alia GB 1,444,936; GB 1,131,989; GB 965,358; GB957260; and GB 972,140. Suitably at least 70 vol. % of the carbondioxide present is removed from the recycle stream, preferably 80 vol.%, more preferably 90 vol. %. Suitably, between 50 and 90 vol. % of therecycle stream is recycled to step i) of the process, preferably between60 and 80 vol. %, in order to get an optimum balance between optimumcarbon use, process efficiency and inert removal.

1. A process for the production of hydrocarbons from gaseoushydrocarbonaceous feed comprising: i) partially oxidating the gaseoushydrocarbonaceous feed with oxygen containing gas at elevatedtemperature and pressure into synthesis gas; ii) catalyticallyconverting the synthesis gas of step i) using a cobalt on zirconiacarrier based Fischer-Tropsch catalyst into a hydrocarbon comprisingstream; iii) separating the hydrocarbon comprising stream of step ii)into a hydrocarbon product stream and a recycle stream; and, iv)removing carbon dioxide from the recycle stream and recycling the carbondioxide depleted recycle stream to step i).
 2. The process of claim 1,wherein the carbon dioxide depleted recycle stream is premixed with thegaseous hydrocarbonaceous feed.
 3. The process of claim 1, wherein partof the recycle stream of step iii) is used as fuel in steam reforming ofgaseous hydrocarbonaceous feed for producing hydrogen supplement forsynthesis gas of step i).
 4. The process of claim 1, wherein part of therecycle stream of step iii) or step iv) is used as fuel for powergeneration.
 5. The process of claim 1, wherein the hydrocarbons productstream is subjected to catalytic hydrocracking.
 6. The process of claim1, wherein the hydrocarbon product stream comprises between 17 and 27 wt% C₁₀-C₁₄.
 7. The process of claim 2, wherein part of the recycle streamof step iii) is used as fuel in steam reforming of gaseoushydrocarbonaceous feed for producing hydrogen supplement for synthesisgas of step i).
 8. The process of claim 2, wherein part of the recyclestream of step iii) or step iv) is used as fuel for power generation. 9.The process of claim 2, the hydrocarbons product stream is subjected tocatalytic hydrocracking.
 10. The process of claim 2, wherein thehydrocarbon product stream comprises between 17 and 27 wt % C₁₀-C₁₄. 11.The process of claim 1, wherein the hydrocarbon product stream comprisesbetween 22 wt % and 27 wt % C₁₀-C₁₄.
 12. The process of claim 3, whereinpart of the recycle stream stream of step iii) or step iv) is used asfuel for power generation.
 13. The process of claim 3, wherein thehydrocarbons product stream is subjected to catalytic hydrocracking. 14.The process of claim 3, wherein the hydrocarbon product stream comprisesbetween 17 and 27 wt % C₁₀-C₁₄.
 15. The process of claim 4, wherein thehydrocarbons product stream is subjected to catalytic hydrocracking. 16.The process of claim 4, wherein the hydrocarbon product stream comprisesbetween 17 and 27 wt % C₁₀-C₁₄.
 17. The process of claim 5, wherein thehydrocarbon product stream comprises between 17 and 27 wt % C₁₀-C₁₄.